What does “Use the pinned node tree” do?
The pinned node tree feature enables you to stick a node tree (pin it) to one instance of the node editor, so that you can open another node editor displaying another node tree.

This feature is really useful when you need to simultaneously compare and edit Node Trees belonging to different data blocks (materials for instance).

Reference: http://wiki.blender.org/index.php/Dev:Ref/Release_Notes/2.67/Node_Editor